OPHIURANS OF THE PHILIPPINE AND ADJACENT WATERS. 333 



border and slightly divergent sides. Beyond the middle of the arms 

 these plates become a little longer than broad. They are all broadly 

 in contact. Between the first and the second under arm plate there 

 is a pair of rather well-developed pores which are obliquely elongated 

 and identical with those which occur in O. affinis. A second pair of 

 pores which are smaller, close together, and rounded occurs between 

 the second and the third under arm plates. 



The side arm plates, which do not project outward, bear five flat- 

 tened spines, which are sometimes appressed against the arms and 

 sometimes more or less divergent, especially the dorsal spines. All 

 these spines are longer than the segment ; the first ventral spine, which 

 is longer than the others, even equals a segment and a half. These 

 spines are inserted on the distal border of the side arm plates. 



The two tentacle scales are oval, elongated, and rather narrow. 

 The external, which is a little smaller than the internal, does not 

 overlap the base of the first ventral spine. 



The color of the specimen in alcohol is very light yellowish gray. 

 The dorsal surface of the disk shows numerous small dark brown 

 spots; the arms show spaced annulations of a rather light brown 

 which involve a variable number of segments, on the average from 

 six to ten, and the length of these darker regions is always greater 

 than that of the light areas which separate them. The ventral 

 surface is light gray. 



Affinities and distinctive features. — The species of the genus 

 Ophiarachna at present known are four in number; they are 0. 

 affinis Liitken, with which it is necessary to unite 0. clavigera 

 Brock, O. incrassata Lamarck, 0. mauritiensis Loriol, and O. robil- 

 lardi Loriol. 



The new species can not be confused with any of these. In the 

 first place all four normally show a supplementary mouth shield 

 which is lacking in O. quinquespinosa. Regarding the other char- 

 acters, our species comes closest to 0. affinis, but it differs from it in 

 the more numerous lateral mouth papillae, in the naked radial shields, 

 in the thicker arm spines, and in the completely different color. 

 Ophiarachna robillardi, which has the radial shields naked as in 

 O. quinquespinosa, has the mouth shields much elongated and the 

 arm spines, which at first are five in number but which rapidly fall 

 to four and then to three, fine and slender. Ophiarachna mauri- 

 tiensis has six or seven very fine and slender arm spines. 



It remains to be seen whether our species is justifiably referred 

 to the genus Ophiarachna. It is particularly on account of the 

 length of the arm spines that I have thought it necessary to refer 

 it to this genus. I believe that there can be little if any hesitation 

 as between the two genera Ophiarachna and Pectinura, the latter 

 being understood in the restricted sense given it by H. L. Clark, 



